Method for producing bisphenols

ABSTRACT

The present invention provides crystals of an adduct of a bisphenol and a phenol and methods of producing those crystals. The crystals may find use in preparing bisphenols.

FIELD OF THE INVENTION

The present invention provides crystals of an adduct of a bisphenol andphenol, a process for preparing these crystals and a process forpreparing bisphenols.

BACKGROUND OF THE INVENTION

Bis(4-hydroxyaryl)alkanes, in the following called bisphenols, areimportant as starting materials or as intermediates for preparing anumber of commercial products. Bisphenols can be prepared by thecondensation of phenols and carbonyl compounds. Substituted phenols orunsubstituted phenol may be used.

The condensation product from the reaction between phenol and acetone,2,2-bis(4 hydroxyphenyl)propane (bisphenol A, BPA,) is of particularindustrial importance. BPA is used as a starting material for preparingvarious types of polymer materials such as, for example, polyarylates,polyetherimides, polysulfones and modified phenol/formaldehyde resins.Preferred areas of application are the preparation of epoxy resins andpolycarbonates.

Processes for preparing bisphenols by acid-catalysed reaction of phenolswith carbonyl compounds are known, for example from U.S. Pat. No.2,775,620 and from EP-A-0 342 758.

Bisphenols of general structure can be prepared by processes which areanalogous to the preparation of BPA.

Bisphenols can be prepared via an adduct of bisphenol and phenol as anintermediate stage.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the object of providing an improvedprocess, as compared with the prior art, of preparing bisphenols. Theprocess according to the invention is intended to provide in particularbisphenol of high purity.

This object is achieved by a process for preparing crystals of an adductof a bisphenol and a phenol comprising

a) the preparation of a solution containing bisphenol and phenol and

b) continuous performance of the crystallisation in one or morecrystallisation devices which contain a crystallisation tank, acirculating pump and a cooler,

wherein the residence time of the liquid being crystallised in thecrystallisation tank is 2 to 12 hours and

wherein crystallisation is performed in 1 to 5 stages, each consistingof the crystallisation devices mentioned and

wherein the temperature in the crystallisation tank in the lastcrystallisation stage is 40 to 70° C. and

wherein the concentration of bisphenol in the inflow to the firstcrystallisation stage is 15 to 40 wt. % and wherein, in the outflow fromthe last crystallisation stage, the concentration of bisphenol indissolved form in the mother liquor is 10 to 20 wt. % and the proportionof crystallised solids in crystals of the adduct of a bisphenol and aphenol is 20 to 30 wt. %.

Accordingly, the present invention provides this process.

Furthermore, the object according to the invention is achieved by aprocess for preparing a bisphenol comprising preparing the adduct of abisphenol and a phenol by the process according to the invention andobtaining the bisphenol from the adduct of the bisphenol and the phenol.

Accordingly, the present invention also provides this process.

Furthermore, the present invention provides crystals of the adduct of abisphenol and a phenol obtainable by the process according to thepresent invention.

The process according to the invention is performed in such a way thatthe temperature in the crystallisation tank in the last crystallisationstage is 40 to 70° C., preferably 40 to 50° C., in particular 40 to 43°C.

The process according to the invention is performed in such a way thatthe concentration of bisphenol in the inflow to the firstcrystallisation stage is 15 to 40 wt. %, preferably 15 to 35 wt. %, inparticular 25 to 35 wt. %.

As a result of the process according to the invention for preparingcrystals of adducts of bisphenols and phenols, these crystals areobtainable in a form and a purity which has not been disclosed in theprior art.

The process according to the invention for preparing crystals of adductsof bisphenols and phenols has many advantages. Crystals are obtainedwhich produce a bisphenol of such high quality that, after filtrationand removal of the phenol, it can be used without further purificationto produce high quality secondary products, for example polycarbonates,epoxide resins, formaldehyde resins, etc.

The process according to the invention for preparing crystals of adductsof bisphenols and phenols, and thus the process according to theinvention for preparing bisphenols, also has the following advantage. Itprovides adducts of bisphenols and phenols and the bisphenol in suchhigh purity that the further purification steps which are usuallyrequired to prepare a bisphenol suitable for high quality polycarbonatescan be avoided. The mentioned further purification steps which areusually required are, for example, additional crystallisation steps oradditional distillation steps.

Crystals of an adduct of a bisphenol and a phenol according to theinvention have many advantages. They are readily filterable and havehigh purity. They have a low concentration of included impurities.Therefore the adduct of a bisphenol and a phenol is obtained in highpurity following separation of the crystals by filtration.

If the bisphenol is recovered from the crystals of an adduct of abisphenol and a phenol according to the invention, then this provides anovel, advantageous process for preparing bisphenols.

The present invention has many advantages. The process according to theinvention is simple and thus cost-effective. Additional purificationsteps are not required. The products according to the invention arecharacterised by high quality, which is expressed by a low colour index,a high storage stability and a high thermal stability.

Any bisphenols at all may be used according to the invention. Thepreferred bisphenol according to the invention is bisphenol A.

Any phenols at all may be used according to the invention. The preferredphenol according to the invention is unsubstituted phenol.

The process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol is preferably performed in one tofive steps, particularly preferably one to three steps and veryparticularly preferably two steps. In the first step, temperatures ofpreferably less than 100° C., preferably less than 70° C. are used andin the last step, temperatures of preferably less than 70° C.,particularly preferably less than 50° C., very particularly preferablyless than 43° C. are used.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, a circulating crystallisationprocess preferably takes place. Each crystalliser step is preferablyoperated at 20 to 40 times, particularly preferably 25 to 35 times, inparticular 30 times the rate of circulation, with respect to thethroughput. According to the invention, a crystalliser step isunderstood to be a crystallisation device which contains acrystallisation tank, a circulating pump and a cooler. The rate ofcirculation is defined as the amount conveyed through the circulatingpump per unit of time divided by the total amount supplied forcrystallisation per unit of time.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, the procedure is preferably suchthat the circulating stream in the crystalliser is supplied tangentiallyat the foot of the crystalliser and is withdrawn centrally at the headof the crystalliser.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, the procedure is preferably suchthat the reaction mixture is cooled to temperatures below 80° C.,particularly preferably to temperatures below 75° C., in particular 70°C., before crystallisation.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, the reaction mixture from which theadduct is crystallised is preferably mixed into the circulating streamdirectly upstream of the crystalliser.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, the procedure is such that the speedof flow up the crystalliser tank is 0.5 to 4, particularly preferably 2to 3, very particularly preferably about 3 m per minute. The speed offlow upwards is the average speed of the liquid in the crystallisationtank, moving from bottom to top.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, the crystallisation tanks arepreferably operated so that they are full of liquid.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, a circulating pump is preferablylocated above the crystallisation tank.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, the suspension of reaction mixtureand adduct crystals contained therein is preferably circulated with aspecific pump energy of at most 150 Watts/m³, particularly preferably atmost 100 Watts/m³.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, the suspension preferably passesthrough the circulation cooler from top to bottom with a speed ofpreferably 1 to 5 m/sec, particularly preferably 2 to 4 m/sec, veryparticularly preferably 2.8 to 3.2 m/sec.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, the circulation cooler preferablycontains electropolished tubes, in which the suspension is circulated.The surfaces of these electropolished tubes which are in contact withthe suspension have a roughness which is preferably less than 1.5 μm,particularly preferably less than 1 μm. This has the advantage that longoperating times are achieved for the circulation cooler, because onlysmall amounts of deposit are formed on the surfaces.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, the circulation cooler is preferablyoperated with constant temperature warm water as a cooling medium. Inthis case, it is particularly preferable that the temperature differencebetween the constant temperature warm water and the suspension beingcooled is 2 to 6 K, very particularly preferably 3 to 4 K.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, coatings are preferably removed fromthe internal surfaces of the suspension-carrying tubes in thecirculation cooler at regular intervals of preferably 40 days by rapidheating to about 80° C. These coatings may consist, for example, ofbisphenol and of the adduct of a bisphenol and a phenol.

In the process according to the invention for preparing crystals of theadduct of a bisphenol and a phenol, the crystalliser and the associatedperipheral equipment is preferably cleaned at regular intervals ofpreferably 40 to 300 days by heating to preferably 80° C., wherein thecirculation circuit continues to operate.

The preferred process according to the invention for preparing crystalsof an adduct of a bisphenol and a phenol is suspension crystallisationin which the crystals of the adduct of a bisphenol and a phenol areobtained by cooling the reaction mixture.

The process according to the invention for preparing BPA is preferablybased on the acid-catalysed reaction of phenol with acetone, wherein aratio by amounts of phenol:acetone of greater than 5:1 is preferablyused in the reaction. Homogeneous or heterogeneous Brönsted acids orLewis acids are used as acid catalysts, that is, for example, strongmineral acids such as hydrochloric acid or sulfuric acid. Gel-like ormacroporous sulfonated cross-linked polystyrene resins (acid ionexchangers) are preferably used. The details given below refer to aprocess of preparation using acid ion exchangers as catalysts.

In order to produce high selectivity, the reaction of phenol withacetone can be performed in the presence of suitable mercapto compoundsas cocatalysts. These may either be dissolved homogeneously in thereaction solution or be fixed to the sulfonated polystyrene matrix viaionic or covalent bonds. The reaction unit is preferably a fixed layerbed or a fluidised bed which is traversed upwards or downwards or acolumn of the reactive distillation column type.

During the reaction of phenol with acetone in the presence of acidcatalysts and mercapto compounds as cocatalysts, a product mixture isproduced which contains, in addition to unreacted phenol and optionallyacetone, primarily BPA and water. In addition, there are also smallamounts of typical secondary products of the condensation reaction suchas, for example, 2-(4-hydroxyphenyl)-2-(2-hydroxyphenyl)propane(o,p-BPA), substituted indenes, hydroxyphenyl indanoles, hydroxyphenylchromanes, substituted xanthenes and higher condensed compounds withthree or more phenyl rings in the molecular structure.

The secondary products mentioned, such as also water, phenol andacetone, may impair the suitability of BPA for preparing polymers andare preferably separated by suitable methods. High specificationsrelating to purity are generally required for the raw material BPA, inparticular when preparing polycarbonate.

The working up and purification of BPA is normally performed by means ofa multistage cascade of suitable purification processes such as, forexample, suspension crystallisation, melt crystallisation, distillationand desorption. In a preferred embodiment, BPA is isolated from thereaction mixture in the form of an approximately equimolar crystallineadduct with phenol by cooling the reaction mixture, when the BPA/phenoladduct crystallises out. The crystallisation process is preferablyperformed as a suspension crystallisation. Suspension crystallisation isunderstood to be crystallisation from a liquid due to cooling, whereinthe crystals form a suspension with the liquid. The BPA/phenol adductcrystals are then separated from the liquid phase, using equipmentsuitable for solid/liquid separation such as a rotary filter or acentrifuge, and if required taken on for further purification. Thus,adduct crystals are obtained which typically have a purity greater than99 wt. % of BPA, with respect to the secondary components, with a phenolcontent of about 40 wt. %. Impurities which adhere to the surface of theadduct crystals can be removed by washing with suitable solutions whichtypically contain one or more components from the group acetone, water,phenol, BPA and secondary components.

The stream of liquid (mother liquor) produced during solid/liquidseparation contains phenol, BPA, water produced during reaction andunreacted acetone and is enriched in the secondary components typicallyproduced during BPA preparation. In a preferred embodiment, this motherliquor is recycled to the reaction unit. In order to maintain thecatalytic activity of the acid ion exchanger any water produced ispreferably first removed by distillation, wherein any acetone stillpresent is also optionally removed from the mother liquor. The dewateredreaction stream obtained in this way is preferably topped up with phenoland acetone and returned to the reaction unit. Alternatively, water andacetone may also be partly or entirely removed by distillation prior toperforming suspension crystallisation of the BPA/phenol adduct. Duringthe distillation steps described above, some of the phenol present inthe reaction solution may also be removed by distillation.

In the case of this type of circulation procedure the problem is thatsecondary products from the preparation of BPA accumulate in thecirculation stream and can lead to deactivation of the catalyst system.In order to avoid excessive accumulation of secondary components in thecirculation stream, some of the circulation stream, optionally afterpartial or complete recovery of phenol by distillation, is preferablyexcluded from the process chain as a BPA resin.

In addition, it has proven advantageous to pass some or the entireamount of the circulation stream, after solid/liquid separation andbefore or after the removal of water and residual acetone, over arearrangement unit filled with acid ion exchanger. This unit isgenerally operated at a higher temperature than the reaction unit. Inthis rearrangement unit, under the conditions present therein, some ofthe secondary components from BPA preparation and present in thecirculation stream are isomerised to give BPA, so that the overall yieldof BPA can be increased.

The BPA/phenol adduct crystals obtained after the completion ofsuspension crystallisation of the reaction solution and solid/liquidseparation as described above are then taken, if required, to furtherpurification stages, wherein the removal of most of the phenol isachieved.

Thus, the adduct crystals can be recrystallised, for example, fromphenol, from organic solvents, from water or from mixtures of thecompounds mentioned in accordance with a suspension crystallisationprocedure. The phenol present in the adduct crystals can also beentirely or partly removed by choosing a suitable solvent. The phenoloptionally remaining in the BPA after recrystallisation can then beentirely removed by suitable distillation, desorption or extractionprocedures.

Alternatively, phenol can also be removed first from the adductcrystals. Preferred methods for this are desorption of the melt usinghot inert gases, vacuum distillation or a combination of the methodsmentioned. In this way, it is possible to obtain BPA with a residualphenol concentration of less than 100 ppm from the adduct crystals. Bymeans of suitable reaction management and optionally the addition ofstabilisers, it can be ensured that BPA does not decompose to a markedextent under the thermal stresses experienced during the removal ofphenol by distillation or desorption.

Depending on the process conditions for suspension crystallisation fromthe reaction solution and when performing the solid/liquid separationand crystal washing, the BPA obtained is suitable for preparing polymermaterials after the removal of phenol from the adduct crystals. It maybe necessary to take the BPA obtained after the removal of phenol to afurther purification operation, in particular in order to prepare highquality materials such as polycarbonates. Final purification may beperformed by suspension crystallisation from water or suitable organicsolvents, melt crystallisation in the form of a static or dynamic layercrystallisation, extraction with water, aqueous neutral, acid or basicsalt solutions or suitable organic solvents or in the form of asingle-stage or multi-stage distillation. It is possible to obtain BPAwith a purity of greater than 99.5 wt. % by performing the purificationoperations mentioned, or a suitable combination thereof, this BPA beingparticularly suitable for preparing high quality polymer materials.

In the following, a preferred embodiment of the invention is explainedusing a diagram (FIG. 1). The invention is not restricted to thispreferred embodiment.

FIG. 1 shows a two step device for preparing crystals of adducts ofbisphenol and phenol. Supplying the reaction mixture which containsbisphenol and phenol to the device is achieved via pipe 1. This opensinto pipe 2 which supplies the circulating stream from heat exchanger 3to the first crystallisation tank 4. The suspension is thus suppliedtangentially at the foot of the crystallisation tank. The suspension ofreaction mixture and crystals of the adduct of bisphenol and phenol iswithdrawn laterally at the head of the crystallisation tank, throughpipe 5, and supplied to heat exchanger 3 via circulating pump 6. Thesuspension is taken from the first crystallisation tank via pipe 7 andsupplied to the second crystallisation step consisting of a secondcrystallisation tank 8, a second heat exchanger 9 and a secondcirculation pump 10. Finally, the suspension is taken out of the secondcrystallisation tank 8 via pipe 11 for further working up.

In the following, the invention is explained by means of an example,without being restricted to this.

EXAMPLE 1 According to the Invention

In order to work up and purify BPA, the reaction mixture from thepreparation of BPA was supplied to suspension crystallisation equipmentin accordance with FIG. 1. For this purpose, a 2 step crystallisationwas performed, starting at 56° C. in the 1st step and then at 41° C. inthe 2nd step, wherein the bisphenol content of the reaction solutionbeing crystallised was 30%. The reaction mixture at about 70° C. wasmixed into the circulating stream directly upstream of thecrystallisation tank and an upwards speed of flow of about 3 m/min wasmaintained in the crystallisation tank.

Circulation crystallisation was operated at 30 times the rate ofcirculation (with respect to throughput) per crystalliser step and thecirculating stream was supplied tangentially at the foot of thecrystallisation tank and withdrawn centrally at the head of thecrystallisation tank.

The circulation coolers were supplied from top to bottom at a speed of 3m/s through electropolished tubes (advantage: long operating times forthe crystalliser/circulation cooler due to the presence of only smallamounts of deposit on the cooling surfaces) (surface roughness 1 μm) andthe cooler was operated with constant temperature warm water. Themaximum temperature difference from the product side was 3 to 4 K.

The circulation pump was located above the crystallisation tank upstreamof the circulation cooler and was operated with a specific pump energyof at most 100 Watts/m³ of suspension.

The final mixed crystal suspension was withdrawn laterally at the headof the crystallisation tank after a total residence time of 4 hours.

In order to remove BPA and BPA/phenol coatings, the internal surfaces ofthe cooling tubes were cleaned at regular intervals by rapidly heatingup to 80° C. and the entire crystalliser system was cleaned at regularintervals by heating up the product contents and continuing to operatethe circulation circuit.

Bisphenol A/phenol adduct crystals of high purity can be obtained bythis type of crystallisation.

After filtration and separation of the phenol, bisphenol A with a purityof 99.5% and a colour index of 20 Haze was obtained. The Haze colourindex was determined visually by comparing with APHA standardcolorimetry solutions. The value is the number of mg of platinum [aspotassium hexachloroplatinate(IV) with cobalt(II) chloride hexahydratein a ratio of 1.246:1 dissolved in 1000 ml of aqueous hydrochloric acid]which has the same colour as the sample, with the same layer thickness.

The bisphenol A produced in this way can be reacted by a conventionalprocess to give high purity polycarbonate. A 14% strength aqueoussolution of sodium bisphenolate was prepared from the bisphenol A sampleobtained in example 1 (purity >99.5%, colour index 20 Haze) by addingsodium hydroxide (6.5% in water) with the exclusion of oxygen. Thissolution was reacted with phosgene and phenol in the phase interfaceprocess. After working up, a polycarbonate with a relative solutionviscosity of 1.297 was obtained. The YI (yellowness index) of thepolycarbonate was 2.3, the light transmission (ASTM D 1003) was 87.88.

The yellowness index YI was measured in accordance with ASTM D 1925, thetransmission in accordance with ASTM D 1003. The relative solutionviscosity was determined at 25° C. using a solution containing 5 g ofpolymer per liter in dichloromethane.

EXAMPLE 2 Comparison Example

In order to work up and purify BPA, the reaction mixture from thepreparation of BPA was supplied to suspension crystallisation equipmentin accordance with FIG. 1. For this purpose, a 2 step crystallisationwas performed, starting at 56° C. in the 1st step and then at 41° C. inthe 2nd step, wherein the bisphenol content of the reaction solutionbeing crystallised was 30%. The reaction mixture at about 70° C. wasmixed into the circulating stream directly upstream of thecrystallisation tank and an upwards speed of flow of about 3 m/min wasmaintained in the crystallisation tank.

Circulation crystallisation was operated at 5 times the rate ofcirculation (with respect to throughput) per crystalliser step and thecirculating stream was supplied tangentially at the foot of thecrystallisation tank and withdrawn centrally at the head of thecrystallisation tank.

The circulation coolers were supplied from top to bottom at a speed of0.5 m/s through non-electropolished tubes and the cooler was operatedwith constant temperature warm water. The maximum temperature differencefrom the product side was 3 to 4 K.

The circulation pump was located above the crystallisation tank upstreamof the circulation cooler and was operated with a specific pump energyof at most 50 Watts/m³ of suspension.

The final mixed crystal suspension was withdrawn laterally at the headof the crystallisation tank after a total residence time of 4 hours.

In order to remove BPA and BPA/phenol coatings, the internal surfaces ofthe cooling tubes had to be cleaned at short intervals by rapidlyheating up to 80° C. and the entire crystalliser system had to becleaned at short intervals (1×week) by heating up the product contentsand continuing to operate the circulation circuit.

After filtration and separation of the phenol from the bisphenolA/phenol adduct crystals obtained in this way, bisphenol A with a purityof 99.3% and a colour index of 50 Haze was obtained. The bisphenol Aobtained in this way cannot be processed by conventional processes togive extremely pure polycarbonate.

A 14% strength aqueous solution of sodium bisphenolate was prepared fromthe bisphenol A sample obtained in comparison example 2 (purity 99.3%,colour index 50 Haze) by adding sodium hydroxide (6.5% in water) withthe exclusion of oxygen. This solution was reacted with phosgene andphenol in the phase interface process. After working up, a polycarbonatewith a relative solution viscosity of 1.297 was obtained. The YI(yellowness index) of the polycarbonate was 3.1, the light transmission(ASTM D 1003) was 87.36%.

The YI was determined in the same way as described in example 1.

The foregoing examples of the present invention are offered for thepurpose of illustration and not limitation. It will be apparent to thoseskilled in the art that the embodiments described herein may be modifiedor revised in various ways without departing from the spirit and scopeof the invention. The scope of the invention is to be measured by theappended claims.

What is claimed is:
 1. A process for preparing crystals of an adduct ofa bisphenol and a phenol comprising: (a) preparing a solution containingbisphenol and phenol; and (b) crystallizing continuously said adduct ofbisphenol and phenol in one or more crystallization devices whichcomprise a crystallization tank, a circulating pump and a cooler;wherein, the residence time of the liquid being crystallized in thecrystallization tank is 2 to 12 hours; crystallization is performed in 1to 5 stages, each of said stages containing the one or morecrystallization devices; the temperature in the crystallization tank inthe last crystallization stage is 40 to 70° C.; the concentration ofbisphenol in the inflow to the first crystallization stage is 15 to 40wt. %; in the outflow from the last crystallization stage theconcentration of bisphenol in dissolved form in the mother liquor is 10to 20 wt. % and the proportion of crystallized solids in crystals of theadduct of a bisphenol and a phenol is 20 to 30 wt. %; a suspension ofcrystals of said adduct of bisphenol and phenol is circulated througheach crystallization tank at a specific pump energy of at most 150Watts/m³, and said suspension of crystals of said adduct of bisphenoland phenol is circulated through said cooler having a top and a bottomat a speed of 1 m/s to 5 m/s from said top of said cooler to said bottomof said cooler.
 2. The process according to claim 1, wherein thebisphenol is bisphenol A and wherein the phenol is an unsubstitutedphenol.
 3. The crystals of the adduct of a bisphenol and a phenol madeby the process according to claim
 1. 4. The crystals of the adduct ofbisphenol A and unsubstituted phenol made by the process according toclaim
 2. 5. The process for preparing a bisphenol according to claim 1further comprising recovering the bisphenol from the adduct of thebisphenol and the phenol.
 6. The process for preparing bisphenol Aaccording to claim 2 further comprising recovering the bisphenol A fromthe adduct of bisphenol A and phenol.
 7. The process of claim 1 whereinsaid suspension of crystals of said adduct of bisphenol and phenol iscirculated through each crystallization tank at a specific pump energyof at most 100 Watts/m³, and said suspension of crystals of said adductof bisphenol and phenol is circulated through said cooler at a speed of2 m/s to 4 m/s from said top of said cooler to said bottom of saidcooler.
 8. The process of claim 1 wherein said suspension of crystals ofsaid adduct of bisphenol and phenol is circulated through said cooler ata speed of 2.8 m/s to 3.2 m/s from said top of said cooler to saidbottom of said cooler.
 9. The process of claim 1 wherein the liquidbeing crystallized is circulated through each crystallization tank at acirculation rate, and the liquid being crystallized passes through eachcrystallization tank at a throughput rate, and the ratio of saidcirculation rate to said throughput rate is 30.